I. Field of the Invention:
This invention relates to improved pressure sensitive hot melt adhesives. More specifically, it relates to a novel process for the production of hot melt adhesives having pressure sensitive properties comprising the steps of interpolymerizing acrylic-based copolymers with selected photoinitiators to produce an ethylenically saturated prepolymer which is applied to a substrate and subjected to ultraviolet radiation. The use of this prepolymer enables the material to be applied in hot melt form and, upon exposure to ultraviolet radiation, to cure to provide a high molecular weight pressure sensitive adhesive displaying improved tack, cohesive strength and end use performance now only obtainable from solvent systems.
II. Brief Description of the Prior Art:
Pressure sensitive adhesive compositions comprise a class of adhesives which, when coated on a suitable substrate share the common characteristic of being agressively and permanently tacky at room temperature and are thus able to adhere to surfaces by the application of nothing more than manual pressure.
Hot melt adhesives are solid materials at room temperature. Application of heat melts the solid adhesive bringing it to the liquid or fluid state in which form it is applied to a substrate; on cooling, the adhesive then regains its solid form.
This invention is directed to a process for producing hot melt pressure sensitive adhesives, i.e. adhesives which are applied in the liquid or fluid state at elevated temperatures and, upon cooling, form solid coatings which are permanently tacky and which adhere on mere contact with adhesive receptive surfaces to which they are applied. Adhesives of this type may be applied to various substrates such as paper, cloth and plastic films and the resulting coated substrates may then be converted into tapes and labels which are especially useful in the packaging field for any number of applications involving marking, sealing, reinforcing or bonding or are useful in the pharmaceutical or health field for such applications as bandages.
In preparing a pressure sensitive hot melt adhesive, the desirable properties include a high cohesive strength at room temperature for good holding power, low shrinkage of the adhesive film on flexible plastic substrates and retention of pressure sensitive properties (i.e. tack retention) during storage and use together with a relatively fluid viscosity at commonly employed application temperatures of 120.degree. to 180.degree. C.
Acrylic polymers and copolymers have found extensive use as the base polymers in pressure sensitive adhesive lacquers or water-based systems mainly because of their clarity, adhesion to non-primed surfaces, good aging characteristics, as well as their excellent properties of tack, peel and cohesive strength. Many attempts to utilize acrylic polymers or copolymers in pressure sensitive hot melt adhesives have not proven successful because of difficulties associated with melt viscosity. In order to have a hot melt adhesive that is sufficiently fluid at application temperatures, either a very low molecular weight polymer must be used or a high molecular weight polymer must be extended with a high proportion of low molecular weight oils or resins (tackifiers) to reduce the melt viscosity. Both of these alternatives are undesirable since low molecular weight polymers will produce adhesives which lack cohesive strength and the use of expensive additives will frequently detract from the cohesion or tack of the resultant adhesive.
Previous attempts to render acrylic polymers or monomer/ oligomer systems useful for hot melt applied pressure sensitive adhesives by incorporating therein photoinitiators and subjecting the compositions to actinic radiation have not been successful since it has been difficult to cure the compositions to the proper degree of tackiness. A usual result is an overcured, hard coating possessing little, if any, pressure sensitivity. Moreover, many of the photoinitiators were also sensitive to visible light wavelengths and this sensitivity caused further curing of the adhesive during storage or use thereof. Finally, for many applications such as surgical bandaging, it is necessary that any externally added components (e.g. photoinitiators) be fully bound into the final composition; many of the compositions of the prior art contained photoinitiators which leached out during use.
Attempts have also been made to utilize acrylic monomers sensitive to electron beam radiation to effect crosslinking thereof. These compositions suffer from the obvious disadvantages associated with electron beam bombardment, i.e. the necessary equipment required for producing such hard radiation as well as the cost and dangers associated therewith.
It is therefore an object of the present invention to provide a process for the production of pressure sensitive hot melt adhesives which display excellent properties of tack, tack retention and cohesive strength, yet which are characterized by relatively low melt viscosity for ease of application.
It is a further object to provide a process for the production of pressure sensitive hot melt adhesives which will be cured to the desired molecular weight by exposure to ultraviolet radiation and which which will remain tacky, without further spontaneous curing when exposed to visible light.
These and other objects will be apparent from the description which follows.